Nanofluids are used in a variety of situations, such as in the form of nano-lubricants. Such nanofluids may be engineered by dispersing metallic or nonmetallic nanoparticles in traditional base fluids, such as engine oil, metalworking fluid, water, ethylene glycol, and the like.
The idea of nanofluids, i.e., nanoparticle-fluid dispersion, was introduced by Choi (S. U. S. Choi, “Enhancing thermal conductivity of fluids with nanoparticles,” in Developments and Applications of Non-Newtonian Flows, D. A. Singer and H. P. Wang. Eds., ASME, New York, N.Y., USA, 1995) in the mid 1950's at the Argonne National Laboratory. However more than a century ago, Maxwell (J. C. Maxwell, “A treatise on electricity and magnetism,” oxford: Oxford University Press, 1904) first theoretically proposed his technique to enhance the thermal conductivity of heat transfer fluids by adding highly conductive particles. Suspension of nanoparticles in various fluids, have been extensively investigated for applying in different types of applications in the past decades. Depending on the application, nanofluids are generally classified such as heat transfer nanofluids, anti-wear nanofluids, metalworking nanofluids, coating nanofluids, and chemical nanofluids.
Since the introduction of heat transfer nanofluids by Choi (S. U. S. Choi, “Enhancing thermal conductivity of fluids with nanoparticles,” in Developments and Applications of Non-Newtonian Flows, D. A. Singer and H. P. Wang. Eds., ASME, New York, N.Y., USA, 1995), a number of studies have been conducted on thermal conductivity enhancement of nanofluids. It has been shown by different investigators that nanofluids have significantly improved heat transfer characteristics compared with the base fluids. Therefore, nanofluids have the potential to improve heat transfer and energy efficiency in thermal systems in applications such as microelectronics, power electronics, transportation, nuclear engineering, heat pipes, refrigeration, air-conditioning and heat pump systems (J. C. Maxwell, “A treatise on electricity and magnetism,” oxford: Oxford University Press, 1904; C. H. Li et al., “Experimental investigation of temperature and volume fraction variations on the effective thermal conductivity of nanoparticle suspensions,” J. Appl. Phys. 99 (2006) 084314; X. Q. Wang et al., “Heat transfer characteristics of nanofluids: A review,” Int'l J. Therm. Sci., (2007) 46: 1; S. K. Das et al., “Heat transfer in nanofluids—a review,” Heat Transfer Eng., 27 (2006) (10): 3).
Another application of nanofluids is in the field of lubrication. It has been demonstrated that dispersed nanoparticles in lubricants can be used to improve lubrication and result in lower friction and wear. The mechanisms by which nanoparticles improve lubrication are described in the literature as the formation of a transferred solid lubricant film from molybdenum disulfide (MoS2) nanoparticles under the contact pressure (J. Gansheimer et al., “Molybdenum disulfide in oils and greases under boundary conditions,” ASME, Journal of Lubrication Technology 95 (1973) 242-248), rolling of spherical nanoparticles in the contact zone (L. Cizaire et al., “Mechanisms of ultra-low friction by hollow inorganic fullerene-like MoS2 nanoparticles,” Surface and Coatings Technology 160 (2002) 282-287), reducing asperity contact by filling the valley of contacting surface (L. Rapoport et al., “Tribological properties of WS2 nanoparticles under mixed lubrication,” Wear 255 (2003) 785-793), and shearing nanoparticles at the interface without the formation of an adhered film (J. Gansheimer et al., “A study of solid lubricant in oils and greases under boundary conditions,” Wear 19 (1972) 430-449).
Nanoparticles can be dispersed in metalworking fluids for operations such as drilling, cutting, milling, and grinding. The resultant nanofluids can be used to prevent welding between the workpiece which results in better surface finish of the workpiece and a longer tool life (M. Mosleh et al., “Modification of sheet metal forming fluids with dispersed nanoparticles for improved lubrication,” Wear, Wear 267 (2009) 1220-1225; M. Mosleh et al., “Deaglomeration of transfer film in metal contacts using nanolubricant,” Tribology Transaction, 55 (2012) 52-58).
The use of nanofluids in a wide variety of applications is promising, but poor suspension stability of nanoparticles in the solution hinders the further development of nanofluids applications. For example, as the nanofluid is stored, the nanoparticles settle out, thereby losing the performance improvements desired by inclusion of nanoparticles. It is believed that strong van der Waals interactions between nanoparticles cause aggregation which leads to quick particle settlement as the particles become larger. Once the nanoparticles settle, the nanofluid loses its property enhancement.
FIG. 1 shows the aggregation behavior of Molybdenum disulfide (MoS2) nanoparticles in 10w30 engine oil. Aggregation causes the nanoparticles to form particle clusters after the first few hours. Then, gravity causes the clusters to settle quickly. Therefore, after the first few hours, the average size of remaining particles decreases.
FIG. 2 shows a representation of settlement of diamond nanoparticles of various concentrations in Boelube®, an advanced cutting fluid made by The Orelube Corporation, after 24 hours of being stationary.
To resolve the settlement problem and to attempt to obtain homogeneous stable nanofluids, physical and chemical treatments such as surfactants, applying strong forces on the cluster of the suspended particles, and dispersing agents have been utilized (A. Ghadimi et al., “A review of nanofluid stability properties and characterization in stationary conditions,” International Journal of Heat and Mass Transfer 54 (2011) 4051-4068; Y. Hwang et al., “Stability and thermal conductivity characteristics of nanofluids,” Thermochim, Acta 455 (1-2) (2007) 70-74).
Three different methods are used in the literature in order to manage stability of suspension against settlement of nanoparticles. A first method has attempted to use surfactants. This method has been utilized to keep the suspension stable through modification of hydrophobic surfaces of nanoparticles into the hydrophilic state. A repulsion force between suspended particles is caused by the zeta potential which will rise due to the surface charge of the particles suspended in the base fluid.
A second method has attempted to use pH control. The electrokinetic properties of an aqueous solution can be used to influence the nanoparticle stability. Stabilization of a dispersed suspension can be obtained by a high surface charge density that causes strong repulsive forces. It has been discovered that simple acid treatment brings a better carbon nanotube suspension in water. A hydrophobic-to-hydrophilic conversion of the surface nature, due to the generation of hydroxyl group, is involved in this process. As the pH of the nanofluid increases beyond the isoelectric point, the surface charge increases by using the surfactant. In this regard, the nanoparticles may be more stable in the fluid.
A third method utilizes ultrasonic vibration. Ultrasonic disruptors, ultrasonic baths and homogenizers are useful means for breaking down the agglomerations in nanofluids. These devices are applied by many researchers to obtain homogeneous suspensions with fewer aggregation particles. With fewer agglomerations, it is believed that the nanoparticles may stay in solution longer before settling to the bottom.
Therefore, it is desirable to produce a nanofluid which further maintains the nanoparticles suspended in the fluid.